The present invention relates to image data manipulation. It finds particular application in conjunction with the three dimensional presentations of computerized tomographic image data and will be described with particular reference thereto. However, it is to be appreciated, that the invention will also find application in conjunction with the display of medical diagnostic image data from other imaging modalities, as well as to images from numerous non-medical applications.
Heretofore, CT scanners have irradiated a planar region of a subject from various angles and detected the intensity of radiation passing therethrough. From the angle and radiation intensity information, two dimensional image representations of the plane are reconstructed. A typical image representation includes a 51233 512 pixel array, although smaller and larger arrays are known. In a black and white image, each pixel has a corresponding value or number which is indicative of the gray scale to be displayed at that pixel. For three dimensional imaging, a plurality of slices are generated, e.g. 60 closely adjacent parallel slices, each of which is represented by a 512.times.512 array of pixel values. The pixel values of the multiple slices are treated as a 512 .times.512.times.60 pixel array or three dimensions of image data. Various planes or other surfaces can be defined through the three dimensional data and displayed.
However, viewing a three dimensional object as a series of slices is not natural. Considerable training on the part of the radiologist is required in order to integrate the stack of CT images mentally. Other medical professionals, such as surgeons, who are not trained as radiologists have difficulty performing the mental integration necessary to conceptualize the three dimensional appearance of the object represented by a stack of slice images. Accordingly, techniques have been developed for presenting a three dimensional presentation which allows the inspection of the object along any cutting plane in a natural fashion.
Generally, such three dimensional presentations include a display of only the extended surfaces which a viewer would see and an internal part of the object through the cut of the object by an appropriate plane or surface. To create the illusion of depth, the angle of the tangent to the surface at each point is estimated and shading is added in accordance with the angle of the surface tangent relative to a preselected illumination point. In a black and white CT image, the shading is added by increasing the brightness or whiteness of each pixel value in proportion to how nearly perpendicular it is to the light source and by increasing the black scale in proportion to the degree that the tangential surface faces away from the light source. For example, a gray scale value that is proportional to the sine/cosine of the angle between the tangent surface and the light source may be combined with each pixel value.
To generate the pixel values for display, every pixel value of the three dimensional data was examined. Each data value was examined to determine whether or not it shows in the resultant image. Each data value which does show is assessed relative to the other data values to determine what contribution, if any, it makes to the image. None could be readily dismissed as not showing. Specifically, air produces a pixel value characteristic of black. Because air is transparent to the viewer, values from pixels hidden behind pixels whose values were indicative of air show through, hence must be displayed. Analogously, other types of tissue that have characteristic pixel values or CT numbers could also be defined as transparent and removed from the view semitransparent, or the like. Hence, the location of the pixel within the data alone was not determinative of whether or not the pixel value would show in the image. Rather, each pixel value had to be considered in the context of its surrounding pixels. This was computationally very time consuming. Note that a 512.times.512.times.60 pixel data set contains almost sixteen million pixels. Various tricks have been developed, many of which are application specific for trying to reduce or identify a subset of all available pixels to project up to the cutting surface or viewing screen to determine their contributions.
Once the three dimensional presentation is displayed on the screen, it often proved advantageous to view it from a different direction. For example, a critical surface portion may be partially obscured or it may be necessary to see the backside before starting surgery. For the new viewing direction, the entire process was repeated anew. Effectively, all of the data within the three dimensional volume was rotated to the appropriate orientation relative to the viewing screen, and the contribution of each pixel was projected up to the plane of the screen for reassessment. In the prior art, all of the data was rotated or shifted to achieve the proper location of the screen relative to the data before the data was projected up to the screen. The shifting of almost sixteen million pixels of data and the interpolation of data, where necessary, further increased processing time.